1. Technical Field
This invention relates generally to exhaust gas emission control for internal combustion engines, and more particularly to the control of emissions during cold-start operation.
2. Background Art
An oxygen sensor, often referred to as an O2 sensor, a lambda sensor or an exhaust gas oxygen (EGO) sensor, is one of the most critical sensors on internal combustion engines, particularly fuel-injected gasoline-fueled engines. An oxygen sensor somewhat resemble a spark plug in external appearance and is located in the exhaust manifold upstream of a catalytic converter, preferably in close proximity to an exhaust port. When at operating temperature, an oxygen sensor becomes a miniature battery that generates a voltage based on the differential between the oxygen content of the exhaust gas and the oxygen content of the ambient air. Accordingly, an oxygen sensor can readily provide an electrical signal representative of the amount of oxygen in the exhaust stream to an electronic control unit (ECU) that controls one or more engine parameters such as the air/fuel A/F ratio. Thus, a major benefit of the oxygen sensor is the ability to control, through the signal supplied to the ECU, exhaust emissions such as carbon monoxide, oxides of nitrogen and unburned hydrocarbons.
However, an oxygen sensor must be heated to a temperature of at least about 300xc2x0 C. (about 600xc2x0 F.) before it will start to function, and operates best at a temperature around 750xc2x0 C. (about 1400xc2x0 F.). Before an oxygen sensor reaches operating temperature, typically about 1 to 2 minutes after a cold start, the vehicle electronic control unit runs in what is termed xe2x80x9copen loopxe2x80x9d, where the ECU tosses out the information provided by the oxygen sensor and relies upon preset values to control the air/fuel ratio. This generally results in a fuel-rich state to ameliorate starting problems when the engine is cold.
EPA Federal Test Procedure (FTP75) sets forth the procedure to be used to certify new engine designs, and requires that the engine be run on a simulated driving cycle lasting 2,477 seconds and 11.1 miles. The test procedure starts with a cold-start after an overnight cool down (12 hours) at an ambient temperature of 20-30xc2x0 C. At 20-30xc2x0 C., only about 10% of the components in gasoline are sufficiently volatile and evaporate. Typically, gasoline engines achieve cold-start by massive over fueling, which supplies the xe2x80x9clightest fractionsxe2x80x9d in sufficient quantities for the engine to start from the light fractions alone. In carrying out EPA Federal Test Procedure 75, it has been determined that about 60-80% of the total tailpipe hydrocarbon emissions produced in the course of the test are produced within the first 60-120 seconds after startup of the engine from a cold start.
Therefore, a major source of cold start hydrocarbon emissions is engine misfire due to the inability of gasoline to easily evaporate when sprayed onto a cold engine surface. Typically, the fuel is targeted at the back of the intake valve, because it is generally the hottest surface in the engine intake system. However, the back of the intake valve takes about a minute to heat up once the engine is started. Other sources of high tailpipe hydrocarbon (HC) emissions during cold-start include misfire due to poor air/fuel (A/F) ratio control, the catalytic converter does not xe2x80x9clight-off,xe2x80x9d (i.e., it does not achieve 50% efficiency in reducing pollutants) until several minutes after a cold start, and poor A/F ratio due to open loop control before the warmup of the exhaust gas oxygen (EGO) sensor.
On engines in which the air/fuel ratio is controlled by an oxygen sensor, other control methods must be employed to reduce exhaust emissions upon a cold engine start. For example, in an attempt to overcome poor A/F ratio control during cold startups, U.S. Pat. No. 6,161,531, issued Dec. 19, 2000, to Hamburg, et al. for Engine Control System With Adaptive Cold-Start Air/Fuel Ratio Control, describes an adaptive correction method for adjusting, or modifying, preset control parameters during cold-start through the use of an EGO sensor. The adaptive correction method is an open loop correction based upon a preestablished correction table. More specifically, Hamburg, et al. uses the EGO sensor to correct the table used for cold-start air/fuel ratio control.
Other techniques commonly used for reducing cold-start emissions include heating the fuel mixture to reduce problems associated with initial enrichment, modifications to the engine, fuel gasification (including reforming to COH2) and close-coupling of the catalytic converter to the exhaust port. Other techniques for reducing cold-start emissions include retarding ignition timing, installing traps in the exhaust system, secondary air injection upstream of the catalytic converter, and the use of faster warmup oxygen sensors to reduce the time in open loop control.
The present invention is directed to overcoming the above described problems associated with current methods of controlling exhaust gas emissions during cold engine starts. It is desirable to have an effective closed loop control to regulate the air/fuel ratio under cold-start conditions. It is also desirable to have a method for controlling cold-start emissions by a closed loop system in which sensors used in the closed loop control are adaptively calibrated during normal engine operation.
In accordance with one aspect of the present invention, a method for controlling exhaust gas emissions during cold-start of an internal combustion engine having an ion sensor disposed either within or in close proximity to a combustion chamber of the engine includes introducing a mixture of air and fuel in the combustion chamber, combusting the mixture of air and fuel within the combustion chamber and generating a plurality of positively charged ions having a magnitude representative of the oxygen content of the fuel and air mixture. The amount of ions existing in the combusted mixture of air and fuel is sensed by the ion sensor and a signal is thereby generated having a value representative of the oxygen content of the combusted air and fuel mixture. The signal representative of the oxygen content of the combusted air and fuel mixture is compared with a desired value representative of ions produced in an idealized combusted air and fuel mixture. Any difference between the sensed value of the ions existent in the combusted fuel mixture and the desired value of ions in the combusted mixture is determined, and a signal correlative of the difference between the sensed and desired values is generated. A signal controlling the mixture ratio of air and fuel introduced into the combustion chamber is adjusted in accordance with the generated signal correlative of the difference between the sensed and desired values of ions in the combusted mixture.
Other features of the method for controlling exhaust gas emissions during cold-start in accordance with the present invention include using a spark plug disposed in the combustion chamber as a sensor for sensing the concentration of ions existent in the combusted mixture of air and fuel.
Additional features of the method for controlling exhaust gas emissions during cold-start in accordance with the present invention include heating an oxygen sensor disposed in an exhaust system of the engine to a predetermined functional operating temperature and receiving a signal from the oxygen sensor representative of the oxygen content of exhaust gas discharged from the combustion chamber. The signal received from the oxygen sensor is used to calibrate the signal from the ion sensor.
In another aspect of the present invention, an apparatus for controlling exhaust gas emissions during cold-start of an internal combustion engine includes an ion sensor disposed either within or in close proximity to a combustion chamber, and an engine control unit adapted to receive a signal from the ion sensor that is correlative of the magnitude of ions existent in a mixture of air and fuel combusted within the combustion chamber. The engine control unit is also adapted to compare the value of the signal received from the ion sensor with a desirable value of ions present in a combusted air and fuel mixture for mitigation of undesirable products of combustion in exhaust gases discharged from the engine. The engine control unit is further adapted to generate control signals that are modified in accordance to the difference between the sensed value of ions existent in the combusted fuel mixture and the desired value of ions in the combusted mixture. The apparatus further includes a fuel injector disposed in fluid communication with the combustion chamber and adapted to receive one of the modified control signals generated by the engine control unit and inject fuel into the engine in accordance with the modified control signal.
Other features of the apparatus embodying the present invention include the ion sensor being a spark plug having a tip portion disposed within the combustion chamber and adapted to receive another one of the modified control signals generated by the engine control unit and produce electrical charges within the combustion chamber in accordance with the modified control signal.
Another feature of the method embodying the present invention includes calibrating the signal generated by the ion sensor to bring the value of the ion sensor signal into congruence with a signal generated by an oxygen sensor disposed in the exhaust system of the engine.